An image processing apparatus inputs image signals by a plurality of pixels in synchronism with a predetermined clock; designates a reduction ratio of an image represented by the input image signals; cumulatively adds a value on the basis of the inverse of the designated reduction ratio in synchronism with the predetermined clock and holds a value of predetermined number of lower significant digits of the added value; and thins out and interpolates image signals input by the input means in accordance with whether or not the value held by the addition means exceeds a predetermined value.